In recent years, polymethine compounds have come into wide use, among others, as materials for optical recording media and near-infrared absorbing filters, or as light-to-heat converting agents in materials for plate making utilizing laser beams. In the field of materials for plate making utilizing laser beams, in particular, the demand for compounds which are highly sensitive to laser beams emitted by general-purpose semi-conductor lasers, for example in the laser wavelength range of 780 nm to 830 nm, and are fairly soluble in general-purpose solvents, for example alcohols such as methanol and ethanol, has recently been increasing. Further, it is also important that such compounds be stable and easy to handle and free of impurities possibly producing adverse effects in various fields of application. However, any polymethine compound capable of satisfying such requirements is not known.
Since certain polymethine compounds were disclosed in Zh. Org. Khim. (1978), 14 (10), various investigations have been made concerning compounds similar in structural formula itself to the polymethine compounds of the present invention. According to the known methods of synthesizing such compounds, an indolenium compound represented by the formula (III)
wherein R1 represents an alkyl group, which may optionally be substituted, and Z represents an acidic residue, or an indoline compound represented by the general formula (IV)
wherein R1 represents an alkyl group, which may optionally be substituted, for instance, is condensed with a diformyl compound represented by the formula (V) or a dianil compound represented by the formula (VI)
in a dehydrating organic acid in the presence of a fatty acid salt (cf. e.g. WO 01/07524; Japanese Kokai Publication H10-195319, pages 8-10; J. Org. Chem. 1995, 60, 2394; Japanese Patent Specification No. 3045404, Example 1; German Laid-open Patent Specification DE 3721850; Japanese Kokai Publication S62-36469).
Among them, the method of synthesizing polymethine compounds by reacting an indolenium compound of formula (III) with a dianil compound of formula (VI) is the commonest. In that case, however, the acidic residue Z− is limited in kind from the viewpoint of reaction yield of the product polymethine compound and ease of operation in isolation and purification, among others. Such method is generally used in producing polymethine compounds in which Z is the perchloric acid residue, tetrafluoroboric acid residue or p-toluenesulfonic acid residue. Those cases where Z− is a halogen ion other than I−, in particular where Z− is Cl− or Br', are not yet known.
As for the method comprising reacting an indoline compound of formula (IV) with a diformyl compound of formula (V), a synthesis example in which the counter ion to a polymethine compound is Cl− is disclosed in German Laid-open Patent Specification DE 3721850 (in Example 1). However, the compound obtained by the method of synthesis described in this document differs in structural formula from the polymethine compound of the present invention and no other physical characteristic values than the maximum absorption wavelength (λmax) are described in that document. When a compound having the same structure as that of a polymethine compound (X=Cl) of the present invention was produced by the production method disclosed there, the yield was low and the compound obtained was low in purity and in a hydrated form. Further, the diformyl compound of formula (V) as used therein is poor in storage stability and hazardous (positive in mutagenecity testing) and, therefore, caution is necessary in handling the same and the use thereof as a raw material for commercial scale production is undesirable.
WO 01/07524 discloses, in Example 1, an example of synthesis of a compound whose basic structural formula is the same as that of the polymethine compound of the invention in which the counter ion is Br−. Therefore, the present inventors attempted to synthesize the compound identical in structure to the polymethine compound of the present invention in accordance with the method disclosed in WO 01/07524. However, the compound obtained was in a hydrated form and low in purity and, therefore, the use thereof is greatly restricted, for example it cannot be used in systems on which water exerts an adverse influence.
As for the method comprising reacting an indoline compound of formula (IV) with a dianil compound of formula (VI), Japanese Kokai Publication S62-36469 discloses, in Example 3, an example of the production of a compound differing in basic structural formula from the polymethine compound of the present invention but having Cl− as Z−. When a compound structurally identical to the polymethine compound (X=Cl) of the present invention was produced by the production method disclosed there, the substance obtained was a methanol adduct and was a low-purity compound.
Further, a recent Japanese edition of Aldrich Comprehensive Reagent Catalog (2003-2004) describes a compound (X being Cl) identical in structural formula to the polymethine compound of the present invention. However, this substance is a hydrate and is a compound very low in purity and, therefore, the use thereof is greatly restricted, for example it cannot be used in systems on which water exerts an adverse influence.
The term “solvate” as used herein is a generic one including the hydrate.
As described above, the known compound identical in structural formula to the compound of the invention occurs as a solvate due to the process for production thereof and is a low-purity product and, therefore, the use thereof is greatly restricted. When the solvate-form compound structurally identical to the compound of the present invention is used as a light-to-heat converting agent in plate making by the CTP (computer-to-plate) technique, difficulties are encountered, namely the solution stability is poor, and the light-to-heat conversion efficiency widely fluctuates due to the fact that the purity is not constant.
Meanwhile, no report can be found about the fact that in the case of polymethine compounds, there are great differences in stability in solution and in sensitivity between the solvated form and non-solvated form in spite of the same basic structure of the compound.